1. Field of the Invention
The present invention relates to a television synchronous receiver for use as a television receiver and a VTR video tuner.
2. Description of the Prior Art
Recent years saw a wide-spread use of the so-called electronic tuner employing variable capacitance diodes as tuning elements in television receivers and VTR video tuners. The electronic tuner is advantageous in that being a contactless device, it is free from the trouble caused by defective contacts and because it can be electronically controlled, it can be used conveniently in remote control and other applications demanding multiple functions. However, because of the inherent variation in characteristics of the variable capacitance diodes and the need for an inductance in tuning, the electronic tuner presents problems when one aspires to a non-adjustment manufacturing process and automation.
Therefore, it might be contemplated to employ a synchronous receiving system for the purpose of constructing an easily integratable receiver without using variable capacitance diodes and inductors. While a variety of synchronous receiving systems are available, a carrier recovery system is suitable for the tuning of a synchronous carrier with feeble television signals. This system is known as the Costas loop system.
FIG. 1 is a block diagram showing main parts of a carrier recovery system synchronous receiver utilizing the Costas loop system. Its construction comprises a first synchronous detector 1 for synchronous detection of the in-phase component of a modulated carrier input, a second synchronous detector 2 for synchronous detection of the quadrature component thereof, low pass filters 3 and 4 for low-pass filtering of the outputs of said two synchronous detectors 1 and 2, a phase detector 5 for detecting the phase of the synchronous carrier with respect to the modulated carrier by multiplying the output voltages of said two low pass filters 3 and 4, a low pass filter for filtering the output of said phase detector, a voltage controlled oscillator 7 which is controlled by the output of said low pass filter 6, and a 90.degree. phase shifter for shifting the output of said voltage controlled oscillator by 90.degree..
In this Costas loop synchronous receiver, the in-phase and quadrature signal components available from the first and second synchronous detectors 1 and 2 are added to the phase detector 5. A voltage proportional to the phase error between the receiver input which is the modulated carrier, and the output of the voltage controlled oscillator, which is the synchronous carrier, is obtained from said phase detector and this voltage is fed back to the voltage controlled oscillator 7 so as to reduce the phase error to 0.
By applying this prior art system directly to a television receiver, it is possible to obtain the base band video signal of the desired receiving channel by synchronous detection and, also, the sound intermediate frequency signal but the chrominance signal and sound signal of the lower adjacent channel are also generated at the same time. The chrominance signal and sound signal of the lower adjacent channel find their way as interference signals into the base band video signal.
To solve this problem, it might be contemplated to provide a tuning circuit consisting of a variable capacitance diode and an inductor at the high frequency input but such an arrangement would defeat the initial objective of constructing a receiver without enlisting the help of such elements.